In recent years, worldwide efforts have been made to provide increased energy savings. In the field of automotive technology, the development of a fuel-efficient internal combustion engine has been required for energy saving purposes. The most conspicuous internal combustion engine meeting such a demand is a lean-burn internal combustion engine. In particular, an in-cylinder fuel injection internal combustion engine, which is a lean-burn internal combustion engine, injects fuel directly into a cylinder to stratify an air-fuel mixture, thereby making it possible to achieve combustion at an air-fuel ratio of higher than 40 and reduce the pump loss.
In a lean-burn in-cylinder fuel injection internal combustion engine system, which is described above, the air flow rate is not proportional to the torque. Therefore, the lean-burn in-cylinder fuel injection internal combustion engine system generally uses an electronic throttle for electronically controlling the air flow rate unlike a conventional internal combustion engine system.
For the above lean-burn in-cylinder fuel injection internal combustion engine system, torque-on-demand control is required to provide torque desired by the driver at a wide-range air-fuel ratio. Two types of torque-on-demand control are an air-based type and a fuel-based type.
If air-based torque-on-demand control is exercised, a target torque computation section and a target air-fuel ratio computation section determine a target torque and target air-fuel ratio, respectively, as shown in FIG. 27. A target air flow rate computation section for providing the target torque and target air-fuel ratio computes a target air flow rate. An electronic throttle controls the air flow rate. An air flow rate sensor detects an actual air flow rate. A fuel injection quantity computation section determines the quantity of fuel injection from the actual air flow rate and target air-fuel ratio.
If, on the other hand, fuel-based torque-on-demand control is exercised, a target torque computation section determines a target torque as shown in FIG. 28. A fuel injection quantity computation section then determines the quantity of fuel injection for providing the target torque. A target air computation section computes a target air flow rate from the fuel injection quantity and target air-fuel ratio. An electronic throttle controls the air flow rate. Further, fuel-based torque-on-demand control can be exercised to provide feedback control over the air flow rate in accordance with a value output by an air flow sensor.
Fuel-based torque-on-demand control described above uses an electronic throttle to exercise air flow rate control after fuel injection quantity determination. However, a transmission characteristic exists between the electronic throttle and cylinder. More specifically, a transient phenomenon occurs because it generally takes tens to hundreds of milliseconds for the air flow rate controlled near the electronic throttle to arrive in a cylinder as shown in FIG. 29. In an in-cylinder fuel injection internal combustion engine, on the other hand, fuel injection directly occurs within a cylinder. Therefore, the transmission characteristic of the fuel injection side is smaller than that of the air side because time is merely wasted by intermittent combustion.
Meanwhile, the exhaust pipe for an internal combustion engine is usually provided with a three-way catalyst or a catalyst having a three-way catalytic function as an exhaust gas emission purification system. As shown in FIG. 30, the three-way catalyst efficiently purifies carbon hydride (HC), carbon monoxide (CO), which are reducers, and nitrogen oxide (NOx), which is an oxidant, only in the neighborhood of a theoretical air-fuel ratio. From the viewpoint of exhaust gas emission reduction, it is desirable that the air-fuel ratio for an internal combustion engine be adjusted for the theoretical air-fuel ratio.
As regards a lean air-fuel ratio, there is a correlation between the air fuel ratio and internal combustion engine combustion stability as shown in FIG. 31. It is therefore necessary to control the air fuel ratio for the purpose of providing combustion stability of an internal combustion engine. Thus, when an air flow rate transient phenomenon in a cylinder is considered, the fuel injection quantity needs to be controlled from the viewpoint of exhaust gas emission reduction for theoretical air-fuel ratio or from the viewpoint of internal combustion engine combustion stability for lean air-fuel ratio.
Further, the torque of an internal combustion engine is determined almost conclusively by the fuel injection quantity. Therefore, the torque-response is determined by the transient characteristic of air flow rate. As is obvious from the above description, the most important tasks to be accomplished for an internal combustion engine that provides fuel-based torque-on-demand control are to improve the response to air flow rate control and convergence performance, minimize the variations among mass-produced products, and improve the robustness for changes with time.
A technology disclosed by JP-A No. 2000-97086 provides control over the air-fuel ratio of an internal combustion engine. When the target air flow rate changes, this technology provides delay compensation to improve the air flow rate response within a cylinder by temporarily permitting the throttle opening to overshoot the degree of throttle opening for achieving the target air flow rate. However, this control method cannot exercise air flow rate control if the air flow sensor is faulty because it computes the target throttle opening in accordance with the deviation between the actual air flow rate and target air flow rate.
Another technology disclosed by JP-A No. 6-146950 changes the throttle opening by a predetermined amount, if there is any change in the target air-fuel ratio, to eliminate any inadequate feedback response portion of the actual air flow rate with a view toward response improvement. However, since this control method does not detect the actual air flow rate, it cannot properly respond, for instance, to air density changes at a high altitude and exhibits low robustness for air flow rate control accuracy in relation to various environmental changes such as throttle control sensor and actuator characteristic variations.
In consideration of the problems described above, it is an object of the present invention to provide an internal combustion engine controller that is capable of exercising high-performance air flow rate control of a fuel-based torque-on-demand control type, in-cylinder fuel injection internal combustion engine while providing improved response and convergence and enhanced robustness.